The present invention relates to a roll assembly for guiding a pipe in a pipe extrusion plant as well as to a pipe extrusion plant for production of pipes with adjustable diameter.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Pipe extrusion plants are generally known. The production of plastic pipes involves initial melting of a plastic starting material in an extruder and discharge via a pipe die. The exiting thermoplastic molten tube from the pipe die is cooled under shape constraint and calibrated. Calibration is of great importance as far as pipe dimensions and tensions in the pipe wall are concerned. Plastic pipes are calibrated predominantly on the outside, whereby a vacuum calibration device is typically employed.
The still soft molten tube is drawn shortly after discharge from the die through a calibrator which is mounted in the vacuum tank. This calibrator can be configured as sleeve-type calibrator or disk-type calibrator depending on the thermoplastic material. The required contact pressure upon the calibrator is realized by the pressure differential between the normal pressure inside the pipe and the vacuum in the vacuum tank. The vacuum calibration device includes a vessel which can be sealed vacuum-tight and may be equipped preferably as cooling basin with spray nozzles. In and following the calibration path, the extruded pipes must be cooled down to such a degree as to be sufficiently stable in shape for subsequent loads (for example take-off device, winder). Cooling of the pipes may hereby be carried out in tanks with water baths or spray nozzles.
Heretofore, the production of plastic pipes with varying wall thicknesses or outer diameters required the availability of different calibration devices. As a result, the respective parts (e.g. calibrator) must be exchanged, necessitating a shutdown of the machine and causing downtimes.
German Offenlegungsschrift DE 198 43 340 C2 discloses an adjustable calibration device for different pipe dimensions. This calibration device includes a plurality of lamellae which are distributed in spaced-apart relationship about the circumference on the outer side of the pipe to be calibrated. Viewed in travel direction of the pipe, a plurality of such lamellae rims are arranged within a calibrating station, with gaps being provided between the individual lamellae of the individual lamellae rims.
The vacuum tank and the spray baths must be provided with guides to support the pipes. To date, commercially available installations use rolls and support disks to meet this task. Support disks, which have the desired pipe radius on the side facing the pipe, are normally used in the vacuum tank. The use of such support disks appears necessary because the pipe is still very unstable in this early phase and has a tendency to assume an oval shape without this guiding aid.
A drawback of a tube extrusion plant allowing the production of pipes with different diameter is however the need to also change the pipe guidance during each diameter change and the need to manually conform or exchange the guides. As the center line of a pipe should remain essentially the same for different pipes, rolls for greater pipes must be arranged lower, while rolls for smaller pipes must be elevated.
German Pat. No. DE 196 45 832 C1 discloses an apparatus for guiding extruded components, especially pipes, including a parallelogram of mountings on which rotatably supported rolls can be adjusted so as to enable a continuous centered guiding of pipes with varying diameters.
It would therefore be desirable and advantageous to provide an improved roll assembly and improved pipe extrusion plant to obviate prior art shortcomings and to allow automatic adjustment of a pipe guidance to adapt to varying pipe diameters in a simple manner while yet properly supporting the pipe during production and maintaining the pipe substantially centered.